"http.server" --- HTTP 服务器
*****************************

**源代码：** Lib/http/server.py

======================================================================

这个模块定义了实现 HTTP 服务器（ Web 服务器）的类。

警告:

  不推荐在生产环境中使用 "http.server" 。它只实现了基本的安全检查功能
  。

"HTTPServer" 是 "socketserver.TCPServer" 的一个子类。它会创建和侦听
HTTP 套接字，并将请求调度给处理程序。用于创建和运行服务器的代码看起来
像这样：

   def run(server_class=HTTPServer, handler_class=BaseHTTPRequestHandler):
       server_address = ('', 8000)
       httpd = server_class(server_address, handler_class)
       httpd.serve_forever()

class http.server.HTTPServer(server_address, RequestHandlerClass)

   该类基于 "TCPServer" 类，并会将服务器地址存入名为 "server_name" 和
   "server_port" 的实例变量中。服务器可被处理程序通过 "server" 实例变
   量访问。

class http.server.ThreadingHTTPServer(server_address, RequestHandlerClass)

   This class is identical to HTTPServer but uses threads to handle
   requests by using the "ThreadingMixIn". This is useful to handle
   web browsers pre-opening sockets, on which "HTTPServer" would wait
   indefinitely.

   3.7 新版功能.

The "HTTPServer" and "ThreadingHTTPServer" must be given a
*RequestHandlerClass* on instantiation, of which this module provides
three different variants:

class http.server.BaseHTTPRequestHandler(request, client_address, server)

   This class is used to handle the HTTP requests that arrive at the
   server.  By itself, it cannot respond to any actual HTTP requests;
   it must be subclassed to handle each request method (e.g. GET or
   POST). "BaseHTTPRequestHandler" provides a number of class and
   instance variables, and methods for use by subclasses.

   The handler will parse the request and the headers, then call a
   method specific to the request type. The method name is constructed
   from the request. For example, for the request method "SPAM", the
   "do_SPAM()" method will be called with no arguments. All of the
   relevant information is stored in instance variables of the
   handler.  Subclasses should not need to override or extend the
   "__init__()" method.

   "BaseHTTPRequestHandler" has the following instance variables:

   client_address

      Contains a tuple of the form "(host, port)" referring to the
      client's address.

   server

      Contains the server instance.

   close_connection

      Boolean that should be set before "handle_one_request()"
      returns, indicating if another request may be expected, or if
      the connection should be shut down.

   requestline

      Contains the string representation of the HTTP request line. The
      terminating CRLF is stripped. This attribute should be set by
      "handle_one_request()". If no valid request line was processed,
      it should be set to the empty string.

   command

      Contains the command (request type). For example, "'GET'".

   path

      Contains the request path.

   request_version

      Contains the version string from the request. For example,
      "'HTTP/1.0'".

   headers

      Holds an instance of the class specified by the "MessageClass"
      class variable. This instance parses and manages the headers in
      the HTTP request. The "parse_headers()" function from
      "http.client" is used to parse the headers and it requires that
      the HTTP request provide a valid **RFC 2822** style header.

   rfile

      An "io.BufferedIOBase" input stream, ready to read from the
      start of the optional input data.

   wfile

      Contains the output stream for writing a response back to the
      client. Proper adherence to the HTTP protocol must be used when
      writing to this stream in order to achieve successful
      interoperation with HTTP clients.

      在 3.6 版更改: This is an "io.BufferedIOBase" stream.

   "BaseHTTPRequestHandler" has the following attributes:

   server_version

      Specifies the server software version.  You may want to override
      this. The format is multiple whitespace-separated strings, where
      each string is of the form name[/version]. For example,
      "'BaseHTTP/0.2'".

   sys_version

      Contains the Python system version, in a form usable by the
      "version_string" method and the "server_version" class variable.
      For example, "'Python/1.4'".

   error_message_format

      Specifies a format string that should be used by "send_error()"
      method for building an error response to the client. The string
      is filled by default with variables from "responses" based on
      the status code that passed to "send_error()".

   error_content_type

      Specifies the Content-Type HTTP header of error responses sent
      to the client.  The default value is "'text/html'".

   protocol_version

      This specifies the HTTP protocol version used in responses.  If
      set to "'HTTP/1.1'", the server will permit HTTP persistent
      connections; however, your server *must* then include an
      accurate "Content-Length" header (using "send_header()") in all
      of its responses to clients. For backwards compatibility, the
      setting defaults to "'HTTP/1.0'".

   MessageClass

      Specifies an "email.message.Message"-like class to parse HTTP
      headers.  Typically, this is not overridden, and it defaults to
      "http.client.HTTPMessage".

   responses

      This attribute contains a mapping of error code integers to two-
      element tuples containing a short and long message. For example,
      "{code: (shortmessage, longmessage)}". The *shortmessage* is
      usually used as the *message* key in an error response, and
      *longmessage* as the *explain* key.  It is used by
      "send_response_only()" and "send_error()" methods.

   A "BaseHTTPRequestHandler" instance has the following methods:

   handle()

      Calls "handle_one_request()" once (or, if persistent connections
      are enabled, multiple times) to handle incoming HTTP requests.
      You should never need to override it; instead, implement
      appropriate "do_*()" methods.

   handle_one_request()

      This method will parse and dispatch the request to the
      appropriate "do_*()" method.  You should never need to override
      it.

   handle_expect_100()

      When a HTTP/1.1 compliant server receives an "Expect:
      100-continue" request header it responds back with a "100
      Continue" followed by "200 OK" headers. This method can be
      overridden to raise an error if the server does not want the
      client to continue.  For e.g. server can chose to send "417
      Expectation Failed" as a response header and "return False".

      3.2 新版功能.

   send_error(code, message=None, explain=None)

      Sends and logs a complete error reply to the client. The numeric
      *code* specifies the HTTP error code, with *message* as an
      optional, short, human readable description of the error.  The
      *explain* argument can be used to provide more detailed
      information about the error; it will be formatted using the
      "error_message_format" attribute and emitted, after a complete
      set of headers, as the response body.  The "responses" attribute
      holds the default values for *message* and *explain* that will
      be used if no value is provided; for unknown codes the default
      value for both is the string "???". The body will be empty if
      the method is HEAD or the response code is one of the following:
      "1xx", "204 No Content", "205 Reset Content", "304 Not
      Modified".

      在 3.4 版更改: The error response includes a Content-Length
      header. Added the *explain* argument.

   send_response(code, message=None)

      Adds a response header to the headers buffer and logs the
      accepted request. The HTTP response line is written to the
      internal buffer, followed by *Server* and *Date* headers. The
      values for these two headers are picked up from the
      "version_string()" and "date_time_string()" methods,
      respectively. If the server does not intend to send any other
      headers using the "send_header()" method, then "send_response()"
      should be followed by an "end_headers()" call.

      在 3.3 版更改: Headers are stored to an internal buffer and
      "end_headers()" needs to be called explicitly.

   send_header(keyword, value)

      Adds the HTTP header to an internal buffer which will be written
      to the output stream when either "end_headers()" or
      "flush_headers()" is invoked. *keyword* should specify the
      header keyword, with *value* specifying its value. Note that,
      after the send_header calls are done, "end_headers()" MUST BE
      called in order to complete the operation.

      在 3.2 版更改: Headers are stored in an internal buffer.

   send_response_only(code, message=None)

      Sends the response header only, used for the purposes when "100
      Continue" response is sent by the server to the client. The
      headers not buffered and sent directly the output stream.If the
      *message* is not specified, the HTTP message corresponding the
      response *code*  is sent.

      3.2 新版功能.

   end_headers()

      Adds a blank line (indicating the end of the HTTP headers in the
      response) to the headers buffer and calls "flush_headers()".

      在 3.2 版更改: The buffered headers are written to the output
      stream.

   flush_headers()

      Finally send the headers to the output stream and flush the
      internal headers buffer.

      3.3 新版功能.

   log_request(code='-', size='-')

      Logs an accepted (successful) request. *code* should specify the
      numeric HTTP code associated with the response. If a size of the
      response is available, then it should be passed as the *size*
      parameter.

   log_error(...)

      Logs an error when a request cannot be fulfilled. By default, it
      passes the message to "log_message()", so it takes the same
      arguments (*format* and additional values).

   log_message(format, ...)

      Logs an arbitrary message to "sys.stderr". This is typically
      overridden to create custom error logging mechanisms. The
      *format* argument is a standard printf-style format string,
      where the additional arguments to "log_message()" are applied as
      inputs to the formatting. The client ip address and current date
      and time are prefixed to every message logged.

   version_string()

      Returns the server software's version string. This is a
      combination of the "server_version" and "sys_version"
      attributes.

   date_time_string(timestamp=None)

      Returns the date and time given by *timestamp* (which must be
      "None" or in the format returned by "time.time()"), formatted
      for a message header. If *timestamp* is omitted, it uses the
      current date and time.

      The result looks like "'Sun, 06 Nov 1994 08:49:37 GMT'".

   log_date_time_string()

      Returns the current date and time, formatted for logging.

   address_string()

      Returns the client address.

      在 3.3 版更改: Previously, a name lookup was performed. To avoid
      name resolution delays, it now always returns the IP address.

class http.server.SimpleHTTPRequestHandler(request, client_address, server, directory=None)

   This class serves files from the current directory and below,
   directly mapping the directory structure to HTTP requests.

   A lot of the work, such as parsing the request, is done by the base
   class "BaseHTTPRequestHandler".  This class implements the
   "do_GET()" and "do_HEAD()" functions.

   The following are defined as class-level attributes of
   "SimpleHTTPRequestHandler":

   server_version

      This will be ""SimpleHTTP/" + __version__", where "__version__"
      is defined at the module level.

   extensions_map

      A dictionary mapping suffixes into MIME types, contains custom
      overrides for the default system mappings. The mapping is used
      case-insensitively, and so should contain only lower-cased keys.

      在 3.9 版更改: This dictionary is no longer filled with the
      default system mappings, but only contains overrides.

   directory

      If not specified, the directory to serve is the current working
      directory.

      在 3.9 版更改: 接受一个 *path-like object*。

   The "SimpleHTTPRequestHandler" class defines the following methods:

   do_HEAD()

      This method serves the "'HEAD'" request type: it sends the
      headers it would send for the equivalent "GET" request. See the
      "do_GET()" method for a more complete explanation of the
      possible headers.

   do_GET()

      The request is mapped to a local file by interpreting the
      request as a path relative to the current working directory.

      If the request was mapped to a directory, the directory is
      checked for a file named "index.html" or "index.htm" (in that
      order). If found, the file's contents are returned; otherwise a
      directory listing is generated by calling the "list_directory()"
      method. This method uses "os.listdir()" to scan the directory,
      and returns a "404" error response if the "listdir()" fails.

      If the request was mapped to a file, it is opened. Any "OSError"
      exception in opening the requested file is mapped to a "404",
      "'File not found'" error. If there was a "'If-Modified-Since'"
      header in the request, and the file was not modified after this
      time, a "304", "'Not Modified'" response is sent. Otherwise, the
      content type is guessed by calling the "guess_type()" method,
      which in turn uses the *extensions_map* variable, and the file
      contents are returned.

      A "'Content-type:'" header with the guessed content type is
      output, followed by a "'Content-Length:'" header with the file's
      size and a "'Last-Modified:'" header with the file's
      modification time.

      Then follows a blank line signifying the end of the headers, and
      then the contents of the file are output. If the file's MIME
      type starts with "text/" the file is opened in text mode;
      otherwise binary mode is used.

      For example usage, see the implementation of the "test()"
      function invocation in the "http.server" module.

      在 3.7 版更改: Support of the "'If-Modified-Since'" header.

The "SimpleHTTPRequestHandler" class can be used in the following
manner in order to create a very basic webserver serving files
relative to the current directory:

   import http.server
   import socketserver

   PORT = 8000

   Handler = http.server.SimpleHTTPRequestHandler

   with socketserver.TCPServer(("", PORT), Handler) as httpd:
       print("serving at port", PORT)
       httpd.serve_forever()

"http.server" can also be invoked directly using the "-m" switch of
the interpreter with a "port number" argument.  Similar to the
previous example, this serves files relative to the current directory:

   python -m http.server 8000

By default, server binds itself to all interfaces.  The option
"-b/--bind" specifies a specific address to which it should bind. Both
IPv4 and IPv6 addresses are supported. For example, the following
command causes the server to bind to localhost only:

   python -m http.server 8000 --bind 127.0.0.1

3.4 新版功能: "--bind" argument was introduced.

3.8 新版功能: "--bind" argument enhanced to support IPv6

By default, server uses the current directory. The option
"-d/--directory" specifies a directory to which it should serve the
files. For example, the following command uses a specific directory:

   python -m http.server --directory /tmp/

3.7 新版功能: "--directory" specify alternate directory

class http.server.CGIHTTPRequestHandler(request, client_address, server)

   This class is used to serve either files or output of CGI scripts
   from the current directory and below. Note that mapping HTTP
   hierarchic structure to local directory structure is exactly as in
   "SimpleHTTPRequestHandler".

   注解:

     CGI scripts run by the "CGIHTTPRequestHandler" class cannot
     execute redirects (HTTP code 302), because code 200 (script
     output follows) is sent prior to execution of the CGI script.
     This pre-empts the status code.

   The class will however, run the CGI script, instead of serving it
   as a file, if it guesses it to be a CGI script.  Only directory-
   based CGI are used --- the other common server configuration is to
   treat special extensions as denoting CGI scripts.

   The "do_GET()" and "do_HEAD()" functions are modified to run CGI
   scripts and serve the output, instead of serving files, if the
   request leads to somewhere below the "cgi_directories" path.

   The "CGIHTTPRequestHandler" defines the following data member:

   cgi_directories

      This defaults to "['/cgi-bin', '/htbin']" and describes
      directories to treat as containing CGI scripts.

   The "CGIHTTPRequestHandler" defines the following method:

   do_POST()

      This method serves the "'POST'" request type, only allowed for
      CGI scripts.  Error 501, "Can only POST to CGI scripts", is
      output when trying to POST to a non-CGI url.

   Note that CGI scripts will be run with UID of user nobody, for
   security reasons.  Problems with the CGI script will be translated
   to error 403.

"CGIHTTPRequestHandler" can be enabled in the command line by passing
the "--cgi" option:

   python -m http.server --cgi 8000
